Bent architectural glass is often considered a separate animal from its flat glass counterpart. The reason is simple: to specify bent glass correctly, one needs to be familiar with its unique language. This language encompasses height and width dimensions characteristically associated with flat glass, but bent glass is three-dimensional and getting to the desired shape of the glass is more complicated.
Besides the terminology, manufacturing issues associated with bending must be considered in assessing the quality of the final product.

Starting with Flat Glass
As with the other fabricated products, bent architectural glass is made from flat glass. The quality requirements for flat glass are found in ASTM C1036-01, Standard Specification for Flat Glass. The ASTM C1036 standard contains a terminology section that includes a variety of linear and point blemishes such as scratches, rubs, digs, knots, dirt, stones and gaseous inclusions.
Tolerances on these blemishes and test methods for viewing glass are also presented in the standard. Table 1 of ASTM C1036 is probably the most referenced table in any glass standard. The traditional thickness designations of flat glass are given, along
with a thickness range, cut size length and width tolerances, cut size squareness tolerances and stock sheet tolerances for length and width. Because distortion can occur in flat glass due to non-homogeneous layers of flat glass, the standard includes inspection guidelines to identify the vision interference angle, the angle at which distortion in transmission first occurs.

The Vocabulary
Once the flat glass intended for bending has been inspected, a whole new vocabulary comes into play.
The customer must be able to describe the shape and size of the bent architectural glass so that it is manufactured in accordance to the specification. With a variety of shapes available, this is not always an easy task (see sidebar). There are single bends, consisting of a single radius curved on a single axis, possibly with one or two flat areas adjacent to the curvature. There are also compound bends, comprised of curvatures of one or more radii, curved on two or more axes. In addition, there are multiple bends, consisting of curvatures of two or more radii, all curved on a single axis, with at least two of the curvatures possibly separated by a flat area, and with or without one or more additional flat areas next to the curvatures. Finally, there are serpentine bends, composed of concave and convex curvatures of one or more radii on a single axis with or without flat areas adjacent to the curvatures. The customer should provide a template or production piece to be used in detailing the exact shape required.

Besides height, bent glass measurements include several other descriptors. Radius is a geometric term that is based on a circle. If you measure from the center of the circle to the outside of the glass, you will arrive at the outside radius. The chord dimension is determined by drawing a straight line between two points of an arc. The girth is the distance around the concave or convex surface including flat sections, measured perpendicular to the height.

All of this information and more is contained in ASTM C1464-00, Standard Specification for Bent Glass.
The standard includes tolerances on pockmarks and ring marks, two process surface blemishes that can occur during bending. In addition, the standard presents test methods for determining shape accuracy, twist and crossbend. One way to measure shape accuracy is by placing a template on the concave surface of the bend, then measuring the deviation between the template and the glass.

When one or more of the corners of the glass are out of plane, twist is present. Cross bend is determined by placing a straightedge along the vertical edge on the concave surface of the glass, and then measuring the distance between the glass and straightedge.

ASTM C1464-00 DefinitionsBent glass: Flat glass that has been shaped while hot into a body having curved surfaces.

Single bend: Single radius curved on a single axis, possibly with one or two flat areas tangent to the curvature.

Compound bend: Curvature of one or more radii, curved on two or more axes.

Multiple bend: Curvature of two or more radii, all curved on a single axis, with at least two of the curvatures possibly separated by a flat area, and with or without one or more additional flat areas tangent to the curvatures.

Serpentine bend: Concave and convex curvatures of one or more radii on a single axis with or without flat areas tangent to the curvature.

Other standards
Because bent architectural glass often incorporates low-E coatings or is laminated for safety, the manufacturer must consider other fabricated glass standards, such as ASTM C1376 and ASTM C1172, Standard Specification for Laminated Architectural Flat Glass. Coating blemishes are addressed in ASTM C1376 and laminating blemishes, such as bubbles, delamination, discoloration, hair, lint, interlayer scuff and streaks are covered in ASTM C1172. Considering the fact that one piece of bent architectural glass can face a myriad of quality issues, the only way to handle the matter is through a vigilant inspection process. That is to say, incoming glass must be inspected before it is cut, glass must be inspected after cutting and before bending and after bending before laminating.

Bringing in Quality
Inspecting glass for quality involves training and teamwork. As employees incorporate a regular inspection routine, they are likely to complain that the effort is time-consuming and problematic. No one wants to bring attention to a problem or find another personís mistake. However, it is important to understand that detecting nonconformance is really helping to improve overall quality in the workplace.

Training is vital to teach employees how to detect nonconformance. A supervisor or quality manager is responsible for giving specific instructions to each department in the factory on the inspection process. A team approach is also an effective way of assessing nonconformance issues and proactively generating innovative solutions to management.Satisfying the Customer
To many customers, specifying and detailing bent architectural glass prior to ordering is an intricate and lengthy process. It is worth fully understanding the customerís expectations before attempting to produce the order. Some companies, such as ours, provide technical support to determine the proper dimensions and shape of the glass, and work with templates, tracings and computer-generated drawings to be sure they are on the same page as the customer. Quality standards are useful to transmit key ideas and terms to the customer. It is important that the manufacturer and customer speak the same language so the end result is satisfying, both from a production and design perspective.

Steve Lerner is the president of Bent Glass Design Inc. based in Hatboro, Pa. He served on the ASTM C14.08 Task Group that developed ASTM C1464 and is currently attending ISO TC 160 (Glass in Buildings) Working Group meetings to develop an ISO standard for curved/bent glass.